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Ellipsoidal Artificial Melanin Particles as Building Blocks for Biomimetic Structural Coloration Michinari Kohri, Yuki Tamai, Ayaka Kawamura, Keita Jido, Mikiya Yamamoto, Tatsuo Taniguchi, Keiki Kishikawa, Syuji Fujii, Naozumi Teramoto, Haruyuki Ishii, and Daisuke Nagao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00400 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 6, 2019

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Ellipsoidal Artificial Melanin Particles as Building Blocks for Biomimetic Structural Coloration

Michinari Kohri*†, Yuki Tamai†, Ayaka Kawamura†, Keita Jido†, Mikiya Yamamoto†, Tatsuo Taniguchi†, Keiki Kishikawa†, Syuji Fujii‡,§, Naozumi Teramoto⊥, Haruyuki Ishii¶, and Daisuke Nagao¶

†

Division of Applied Chemistry and Biotechnology, Graduate School of Engineering,

Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan ‡

Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of

Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan §

Nanomaterials Microdevices Research Center, Osaka Institute of Technology, 5-16-1

Omiya, Asahi-ku, Osaka 535-8585, Japan ⊥

Department of Applied Chemistry, Faculty of Engineering, Chiba Institute of

Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan ¶

Department of Chemical Engineering, Tohoku University, 6-6-07 Aoba, Aramaki-aza

Aoba-ku, Sendai 980-8579, Japan Corresponding Author *E-mail: [email protected] (M. K.)

ACS Paragon Plus Environment

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ABSTRACT Inspired by the structural coloration of anisotropic materials in nature, we demonstrate the preparation of structural color materials by the assembly of anisotropic particles. Spherical artificial melanin particles consisting of a polystyrene core and polydopamine shell were stretched asymmetrically to form uniform-sized ellipsoidal particles with different aspect ratios. The aspect ratio and assembly method of the ellipsoidal particles influence the structural coloration, indicating that the particle shape is one of the important parameters for controlling the structural coloration. The discovery of a method to control the structural color using ellipsoidal particles is useful in basic research on structural colors in nature and provides flexibility in material design and extends the application range of structural color materials. INTRODUCTION In recent years, structural color materials have been of interest for their use in various applications and their scientific importance.1–5 The assembly of submicron-sized colloidal particles is one effective method for producing structural color materials.6–9 From previous studies, it is known that the structural colors from the assembly of colloidal particles depends on four parameters. (i) The size of the particles: the structural colors can be easily controlled by changing the particle size10–12 or the distance between the particles.13,14 (ii) The refractive index of the particles: the refractive index of the material or medium greatly influences the structural color, as the structural color is the coloration obtained by the interaction between the light and the submicron-sized structure.15,16 (iii) The arrangement of the particles: it is known that the angular dependence of the structural color changes depending on the difference in the particle arrangement. When the particles form a close packed structure (so-called colloidal crystal structure), angle-dependent structural colors are obtained.17–19 On the other hand, the roughly packed arrays of particles (so-called amorphous structure) produces angle-independent colors.17–19 (iv) The blackness of particles: it is indispensable to control the blackness of a material to produce a highly visible structural color, as submicron-sized colloidal particles usually scatter light. It has been reported that the visibility of structural color can be improved by carbon black doping, which absorbs scattering light.20–22 The use of black colloidal particles with an absorption ability is also useful to create bright colors. Inspired from the structural


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color of peacock feathers, which are produced from melanin granules, we have recently reported on the realization of structural coloration from artificial melanin particles based on polydopamine (PDA).23 Furthermore, using spherical core–shell type artificial melanin particles composed of a polystyrene (PS) core and a PDA shell, the four parameters mentioned above were independently controlled to obtain high visibility structural color.24–28 In nature, there are some examples where anisotropic elements are used to create structural colors. For example, in the coloring of the peacock feather described above, arrays of rod-like melanin granules are the key structure to produce the peacock’s beautiful colors.29,30 Although the biological functions are unknown, it has been reported that colonies formed by the assembly of bacteria with anisotropic shapes produced vivid structural colors.31 Thus, in addition to the aforementioned four parameters for structural coloring using colloidal particles, the "shape" of the particle is focused on as a fifth parameter. Despite the synthesis of anisotropic particles with various shapes advancing,32–34 there are few examples for producing structural colors based on anisotropic particles.35,36 Herein, we demonstrate the creation of structural colors from the assembly of ellipsoidal artificial melanin particles inspired from the color origin of peacock feathers. Ellipsoidal particles with different particle sizes and aspect ratios were prepared by stretching PS@PDA core–shell particles, known as spherical artificial melanin particles (Figure 1a). The effects of various parameters, including particle size, aspect ratio, particle orientation, and assembly method, on the structural coloration were investigated. Particle assemble behavior during pellet sample preparation was also investigated by optical microscopy.


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Figure 1. (a) Schematic representation of the preparation of the spherical and ellipsoidal artificial melanin particles. (b) TEM images of the spherical and ellipsoidal artificial melanin particles. The insets show an enlarged view of a particle. The synthesized products are designated PSx@PDAy core-shell particles (x: diameter of the PS core particles and y: thickness of the PDA shell layer).

EXPERIMENTAL SECTION Materials. Styrene (St) and polyvinyl alcohol (PVA) were obtained from Kanto Chemical. 2,2'-Azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate (VA057) and 2-propanol were obtained from Wako Pure Chemical. St was dried over calcium hydride and distilled at a reduced pressure. All other chemicals and solvents were of reagent grade and were used as received. Measurements. Scanning electron microscopy (SEM) micrographs of the samples were obtained using a scanning electron microscope (JSM-6510A; JEOL). Transmittance electron microscopy (TEM) micrographs were obtained using a transmittance electron microscope (H-7650; HITACHI). The diameters and ζ potentials of the particles were


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measured by dynamic light scattering (Zetasizer Nano ZS; Malvern). Reflection spectroscopy was performed using a spectrophotometer (V-650; JASCO) equipped with a

reflection

spectroscopy

unit

(ARSV-732;

JASCO)

and

a

microscopic

spectrophotometer (MSV-370; JASCO). Photographs of the samples were taken with a digital camera (OM-D; Olympus). The particles were stretched using a tensile tester (AGI; Shimadzu) equipped with a thermostatic chamber (TCE-N300; Shimadzu). The structural color films were prepared using a dip-coater (ND-0407-S4; SDI). Optical microscopic images were acquired using a digital microscope (VHX-500F; KEYENCE). Preparation of the PS@PDA core–shell particles. Monodispersed PS@PDA core– shell particles with different diameters were prepared as described in our previous paper.24,25 Preparation of the ellipsoidal PS@PDA core–shell particles. Ellipsoidal particles were prepared by modifying the work in previously published papers.37,38 An aqueous PVA solution was obtained by adding 60 g of PVA (degree of polymerization: 2000) in 540 mL of deionized water and stirred for 3 days. PS@PDA core–shell particles (53.8 mg) were added to the PVA solution (approximately 270 g) with continuous stirring. The PVA solution containing the particles (0.2 g/g) was cast into a stainless-steel tray (14 × 24 × 1 cm) and allowed to dry to form PVA films. Particles within the PVA films were stretched by a tensile testing machine (120 °C, 1 mm/min). The center parts (2 × 3 cm) of the stretched films were extracted and soaked and shaken overnight in a 2-propanol-water mixture (3:7 v/v). The viscous supernatant was decanted by centrifugation (12,000 rpm, 1 h) and the sedimented particles were redispersed in a fresh isopropanol-water mixture. The dispersion was then centrifuged 4 times (14,500 rpm, 5 min). The sediment was redispersed in distilled water and centrifuged twice (14,500 rpm, 5 min) to form the ellipsoidal artificial melanin particles. Preparation of structural color pellets by a natural drying process. The suspensions of spherical or ellipsoidal PS@PDA core-shell particle (10-20 wt%) were poured onto a silicone rubber plate. Then, the samples were drying at room temperature for 12 hours, producing structural color pellets. Preparation of structural color films by a dip-coating method. The UV-ozone treated glass substrate was immersed in an aqueous dispersion of elliptical PS @ PDA core-shell


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particles (3 wt%). The particles were dip coated onto the glass substrate at 0.1 µm/s at room temperature. Preparation of micron-sized ellipsoidal PS particles. Micron-sized spherical PS particles were synthesized by soap-free emulsion polymerization, as described in previous work.39 St (11.46 g, 110 mmol), 0.1 M NaOH (1 mL, 0.1 mmol), VA-057 (0.414 g, 1 mmol), and deionized water (100 mL) were added to a flask and deoxygenated by Ar for 15 min. The polymerization was conducted at 65 °C with stirring at 300 rpm in an Ar atmosphere. After 10 h, the samples were separated and purified repeatedly by centrifugation (14,500 rpm for 30 min) and redispersed, producing spherical PS particles. Micron-sized ellipsoidal PS particles were prepared in the same manner as described above.

RESULTS AND DISCUSSION As shown in Figure 1b, spherical artificial melanin particles synthesized with 278, 308, and 408 nm diameters were designated as P1 ARX, P2 ARX, and P3 ARX particles, respectively (AR: aspect ratio, X: measured value of the aspect ratio). The obtained spherical artificial melanin particles were dispersed in a PVA solution to allow them to dry to form a particle-containing PVA film. The particle-containing PVA film was stretched in a tensile tester with heating above the glass transition temperature of the PS. Ellipsoidal artificial melanin particles, P1 AR2.7, P2 AR2.6, and P3 AR2.7 particles, were obtained after dissolving the PVA. As shown in the TEM images of Figure 1b, spherical and ellipsoidal artificial melanin particles with relatively the same sizes were successfully obtained. Table S1 shows the measurement results for the minor and major axis lengths of the obtained particles, respectively. In each sample, the sizes of 100 particles were measured using SEM to measure the minor and major axis lengths. From these values, the aspect ratio (the ratio of the minor axis to the major axis) was calculated. First, we investigated the effect of the aspect ratios of artificial melanin particles on the structural coloration. By controlling the stretching ratio of the PVA films containing P1 AR1.0 particles, ellipsoidal particles with different aspect ratios of 1.3, 1.6, 2.4, and 2.7 were prepared. The water dispersion of the resulting particles was dropped and air-dried


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to prepare the structural color pellets. Figure 2a shows the digital microscope images of the structural color pellets together with the TEM image of the particles used. The pellet sample from conventional spherical artificial melanin particles (P1 AR1.0) exhibited a bright orange structural color. As the aspect ratio increased, the color changed from orange to green to blue. The reflection spectra of the pellet samples also suggesting the blueshift of the spectra as the particle aspect ratio increased (Figure 2b). Although the highest reflectance of the spherical particles was approximately 45%, the reflectance of the ellipsoidal particles decreased to below 10%. A sharp decrease in the reflectance is probably due to a reduction in the regularity of the particle arrangement. A detailed analysis of the arrangement of particles is discussed later. Although the reflectance is relatively low, a clear structural color was observed visually. The λmax of the reflection peaks, measured with a microscopic spectrophotometer, are plotted as a function of the aspect ratio of the particles (Figure 2c). The λmax for each of the pellets was 573, 570, 560, 496, and 482 nm, respectively, which corresponded to different colors.

Figure 2. (a) Digital microscope images of the structural color pellets. The insets show the TEM images of the particles used. (b) The normalized reflection spectra of the structural color pellets. (c) The peak position (λmax) of specular reflectance spectra of the structural color pellets containing particles with various aspect ratios.


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Next, using the P1–P3 particles, the influence on the structural color due to the difference in the sizes of the spherical particles before stretching was investigated. By selecting the particle diameter of the spherical particles, structural colors for blue, green, and red were obtained by assembling ellipsoidal artificial melanin particles (Figure 3a). The reflection spectra of the pellets from the ellipsoidal particles also showed the blue, green, and red structural colorations (Figure 3b). While the pellet of AR1.0 spherical particles showed an angle-dependent color, the pellets of AR2.6 and AR2.7 ellipsoidal particles showed angle-independent color. Furthermore, compared with pellets with spherical particles, the pellets with ellipsoidal particles showed a pale structural color. This phenomenon is probably due to the thickness of the PDA layer. The thickness of the PDA layer absorbing the scattered light influences the colorful of the structural color.24 In this experiment, ellipsoidal particles are obtained by stretching the spherical core–shell particles. Thus, the thickness of the PDA shell layer decreases with the elongation of the particles. When the thickness of the PDA layer was estimated from the TEM measurement, the thickness of the PDA thin film was 3 nm for the P2 AR1.0 spherical particles and decreased to 2 nm for the P2 AR2.6 ellipsoidal particles (Figure S1). In Figure 3c, the colors from each pellet are plotted on the International Commission on Illumination (CIE) 1931 chromaticity diagram. It was confirmed that the color of the ellipsoidal particles after stretching shifted to the blue, green, and red regions, respectively. After elongation of the particles, the color plot moved to the inside of the plot, suggesting a decrease in the colorfulness, which supported the above results where the structural color of the pellet became fainter.


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Figure 3. (a) Digital camera images of the structural color pellets from each particle. (b) The reflection spectra of the structural color pellets from the assembly of the ellipsoidal artificial melanin particles. (c) The CIE chromaticity chart of the pellets.

The arrangement of particles influences the structural coloration. We previously reported that a bright structure color film can be obtained by the assembly of spherical artificial melanin particles using a dip coating method.26 In addition to pellet synthesis by a natural drying process, we prepared a structural color film by a dip coating method. Figure 4 shows the digital camera and SEM images of structural colored samples prepared by (a) a natural drying process and (b) a dip coating method, respectively. From the SEM image, we see that the surface of the pellet prepared by natural drying has some vertically oriented deposited particles (Figure 4a). The circle in Figure S2 represents a vertically deposited particle. In contrast, it was shown that almost all the particles were deposited


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in a landscape state on the film surface prepared by the dip coating method, suggesting the high orientation degree (Figure 4b).

Figure 4. SEM images of the structural colored samples prepared by: (a) a natural drying process and (b) a dip coating method. The insets show the digital photo images of the structural colored samples. The order parameters (S) were calculated by Eq. 1.

To investigate the degree of orientation of the ellipsoidal particles on the surface of the samples, the order parameter (S), which is an index indicating a two-dimensional orientation order degree, was calculated by:40,41 1

𝑆 = 2(3⟨cos2 𝜑⟩ ― 1)

(1)

where φ is the angle formed by the orientation principal axis, which is the average orientation direction in which the long axis of the anisotropic particles in the system faces (S = 0 if they are randomly oriented and S = 1 if they are completely vertical).41 Figure S3 shows the method used to measure φ. The order parameters (S), calculated by Eq. 1, for the particles generated by the natural drying process and the dip coating method are summarized in Figure 4. It was confirmed that the order parameter of the particle assembled structures prepared by the dip coating method was higher than the structure prepared by the natural drying method. Figure 5 shows the reflection spectra of the structural colored samples. The reflection spectrum of the film prepared by the dip coating


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method shifted slightly blue compared to the pellet samples. The reflectance also increases, which is believed to be due to an increase in the degree of orientation. The assembly of the ellipsoidal particles with a higher orientation degree is possible via the dip coating method, and as a result, the reflectance increases and the reflection peak is blueshifted.

Figure 5. Reflection spectra of the samples shown in Figure 4. The samples were prepared from (a) P1 AR2.7, (b) P2 AR2.6, and (c) P3 AR2.7 ellipsoidal particles. The numbers in the Figure represent λmax. Black line: natural drying process. Red line: dip coating method.

In the dip coating method, the ellipsoidal particles were oriented in the horizontal direction, and the reflectance was affected. We investigated the direction dependency of the reflectance. With the pulling direction as the x-axis, each of the axes was rotated as a base point to give three patterns of angles (Figure 6a). First, measurements were made using sample of P3 AR2.7 for the case of rotation around the z-axis (rotation angle: α). As shown in Figure 6b and Figure S4a, no change with α was observed for the λmax at each angle, indicating that rotation in the z-axis direction does not affect the reflection characteristics. Next, the sample was rotated around the y-axis (rotation angle: β). While the reflectance decreased, the value of the λmax was almost constant regardless of β (Figure 6c and Figure S4b). If the ellipsoidal particles are fully oriented, a blue shift of the reflection wavelength as seen in spherical particles will be obtained.28 Since the elliptical particles were not completely oriented, it seems that there was no angular dependence


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even when rotating in the y-axis direction. In contrast, when the sample is rotated around the x-axis (rotation angle: γ), the reflectance decreases and the value of the λmax was red shifted (Figure 6d and Figure S4c). Since the rotation around the x-axis was observed from the major axis direction of the ellipsoidal particles, the minor axis length of the particles was apparently increased and the reflection wavelength was red shifted (Figure S4d). It was shown that the angular dependence of the structural color film obtained by the assembly of ellipsoidal particles was different depending on the rotation axis. However, further experiments may be needed, because the particles were not completely orientated in the film this time.

Figure 6. Measurement of the angular dependence of structural colors. (a) Axis setting. Plots of λmax for the reflection spectra of the sample as a function of rotation angle (b) α, (c) β, and (d) γ. Sample: structural color film prepared by dip coating of A P3 AR2.7 particles.

Reflection peaks are usually discussed based on the Bragg-Snell's law:9 mλ =

8 2 2 3𝑑 (𝑛

― sin2 𝜃)

(2)


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where m is the order of diffraction, λ is the wavelength of light, and n is the refractive index of the particles. The n of the particles was set to 1.59, because we have previously reported that n of core-shell particles with PDA shell layers below 5 nm is almost the same as that of core particle.24,25 The value of d is the center-to-center distance between the nearest particles, and θ is the angle between the incident light and the diffraction crystal planes. The d in Eq. 2 is the interparticle distance when spherical particles are used as a component (Figure 7a). Thus, using the interparticle distance d' when an anisotropic particle is assumed as a cylindrical particle, the reflection spectrum of ellipsoidal particles was also calculated using: mλ =

3𝑑 ′2(𝑛2 ― sin2 𝜃)

(3)

Since the ellipsoidal particles were mainly deposited in the landscape state (vide supra), d' is assumed based on the length of the minor axis (Figure 7b). The λmax of the obtained pellet samples from the ellipsoidal particles (AR 2.6 and AR2.7) is shown in Figure 7c together with the theoretical lines calculated by Eq. 2 and Eq. 3. When the particle size was small, the reflection spectrum of the ellipsoidal particles showed a value relatively close to the theoretical line obtained by Eq. 3. As the particle size increased, the deviation from the theoretical line increased. It has been reported that ellipsoidal polymer particles tend to bind at the tip and tip.42,43 In the case of ellipsoidal artificial melanin particles, aggregates bound at the tip and the tip were also observed (Figure S5). Increase of the deviation from the theoretical line may have occurred because the resulting aggregated state is maintained. When the same ellipsoidal particles were assembled by the dip coating method, the value of λmax approached a more theoretical line (Figure 7c □). This is because the particles were closely packed by the dip coating method, as described above. The theoretical and measured values do not completely match. This phenomenon is probably because the shape of the particles was assumed to be cylindrical, in addition to the particles not being close-packed. A more detailed study including a visualization of threedimensional particle arrangement will be necessary.


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Figure 7. Schematic diagrams of the assembly of: (a) the spherical particles and (b) the anisotropic rod-like particles. (c) The λmax of the sample from the ellipsoidal particle (AR2.6 and AR2.7). The line shows the theoretical lines calculated by Eq. 2 (solid line) and Eq. 3 (dotted line).

From the images of the pellets shown in Figure 3a, the shape of pellets formed using the spherical particles (AR1.0) and the ellipsoidal particles (AR2.6 and AR2.7) is clearly different, even though the pellets were produced under the same conditions. The ζ potentials of the spherical particle (ca. -40 mV) and the elliptical particle (ca. -30 mV) were almost the same, and both were well dispersed in water. A donut-shaped pellet sample was obtained from the assembly of the AR1.0 particle, a flat pellet was obtained from the AR2.6 and AR2.7 particles. We believe this phenomenon is probably due to the difference in particle assembly depending on the particle shape. Yunker et al. reported that particle shape affects the drying process for a water dispersion of particles.44 To consider the difference in the shape of the structural color pellets, we investigated the influence of the particle shape on the particle assemble behavior during the natural drying of a water dispersion. Micron-sized PS particles (P4 AR1.0, diameter: 1,090 nm) observable with an optical microscope were prepared, and then stretched in the same manner above to obtain micron-sized ellipsoidal PS particles (P4 AR2.9). Figure 8 shows


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a snapshot of the drying process of the water dispersion of particles (0.23 wt%) on the glass substrate. As water evaporated, the spherical PS particles were assembled closepacked at the edge of the droplet and dried, forming a so-called coffee ring structure. In contrast, the coffee ring effect was suppressed, and the ellipsoidal PS particles were deposited on the entire surface. The ellipsoidal PS particles adsorbed near the air-water interface of the droplets and dried during the formation of a loosely packed structure. This is because the ellipsoidal PS particles were adsorbed near the air-water interface of the droplets and were dried to form a loosely packed structure.44 Differences in the drying process depending on the particle shape will result in different structural color pellet shapes observed in Figure 3 (a).

Figure 8. Microscope images of the drying process for water droplets containing micronsized (a) spherical PS and (b) ellipsoidal PS particles. Particle concentration: 0.23 wt%.

CONCLUSION In summary, we have successfully demonstrated ellipsoidal artificial melanin particles for structural coloration. Ellipsoidal particles with different aspect ratios were prepared by stretching a polymer film containing spherical artificial melanin particles under heating. As the aspect ratio of the elliptical particles increases, the reflection wavelength of the assembled sample of ellipsoidal particles became blueshifted. When the arrangement of the ellipsoidal particles was observed by SEM measurements, these particles were shown to have deposited in a landscape state, suggesting that the reflection


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wavelength was blueshifted depending on the minor axis of the particle. It was also found that the particles were closely packed when using the dip coating method, which enhanced the reflectance. In addition, it was shown that the particle assembly process depends on the shape of the particles, i.e., spherical or ellipsoidal particles, resulting in pellet samples with different shapes. These results indicate that, in addition to the conventional four parameters, such as size, refractive index, arrangement, and blackness of particles, the shape of particles is useful as a fifth parameter to create structural color materials based on the assembly of colloidal particles. These findings, including structural coloration by anisotropic artificial melanin particles, will provide new insights for the development of structural color-based material applications. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX Additional experimental details include particles sizes, microscope images, reflection spectra, and the measuring method of φ (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M. K.) Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS M. K. acknowledges the support of JSPS KAKENHI (Grant Numbers 15H01593 and 17H03110), Iketani Science and Technology Foundation, and a Chiba University Venture Business Laboratory project. REFERENCES


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